In recent years, with the deficiency of radio frequency bands and the increase in communication speed, a modulation system having a high frequency utilization efficiency has been employed, and/or the bandwidth of a transmission signal has been widened. Under these current circumstances, in order to minimize the degradation in signal quality, a power amplifier having a high linearity is preferably used in a transmission section for mobile communication, for example. However, the linearity of an amplifier and power efficiency run counter to each other; therefore, if a wide-band signal is subjected to linear amplification, the power efficiency is reduced, and power consumption increase and/or measures for heat dissipation, for example, will cause problems.
As solutions to the above-mentioned problems, there have been proposed an EER (Envelope Elimination and Restoration) system and an ET (Envelope Tracking) system for controlling a drain voltage or a collector voltage for a power amplifier in accordance with envelope variation of a transmission signal. For example, in the ET system, a drain voltage or a collector voltage supplied to a power amplifier for amplifying a transmission signal is controlled in accordance with envelope information of the transmission signal, thereby increasing the power efficiency.
FIG. 1 illustrates a configuration example of a power amplifying apparatus in which the above-mentioned ET system is used, and the power amplifying apparatus includes: an envelope extraction section 30; a variable power supply section 31; a power amplifier 32; and a delay circuit 33. The envelope extraction section 30 extracts envelope information of a transmission signal, and supplies a voltage control signal to the variable power supply section 31 via a digital-to-analog (D/A) conversion circuit 34. The variable power supply section 31 generates, based on a signal inputted thereto, a drain voltage for the power amplifier 32, and supplies the drain voltage to the power amplifier 32. The transmission signal is delayed for a predetermined period of time by the delay circuit 33, sent to an up-converter 36 via a digital-to-analog (D/A) conversion circuit 35, converted into a carrier frequency signal by the up-converter 36, and amplified as an amplifier input signal by the power amplifier 32.
Japanese Laid-open Patent Application Publication No. 05-136830 discloses an invention in which a drain voltage supplied to an FET element serving as a power amplifier is controlled so as to be compatible with the FET element whose characteristics are varied depending on temperature. Further, Japanese translation of PCT international application No. 2002-522946 discloses an invention in which the maximum peak factor of a transmission signal during a predetermined period is determined, and a voltage supplied to a power amplifier is controlled in accordance with the determined peak factor.
However, even in a power amplifying apparatus in which the above-mentioned ET system is used, as the bandwidth of a transmission signal is widened, the bandwidth of an envelope signal thereof is also widened, which makes it difficult to realize high-speed response of a power supply. Therefore, with the aim of following the average envelope variation instead of following an instantaneous envelope variation, there has been proposed a voltage control method that uses a high-efficiency power supply such as a switching power supply.
FIG. 2 illustrates an example of a power amplifying circuit for performing such a voltage control method. The power amplifying circuit limits the bandwidth of an envelope signal, which is varied at a high speed, by a low-pass filter (LPF) 37, converts the envelope signal into a signal whose variation speed is slow, and supplies a drain voltage to a power amplifier 32 via a variable power supply section 31.
However, the bandwidth limitation performed using the low-pass filter 37 is influenced by previous and subsequent envelope amplitudes; therefore, as illustrated in FIG. 3, a voltage different from an optimal voltage at a peak of the envelope signal is applied to a drain of the power amplifier 32. For example, with respect to the peak position of an envelope signal I within a circle indicated by the dotted line in FIG. 3, the peak position of an output signal II passed through the low-pass filter 37 is largely deviated.
At the peak of the envelope signal, an input signal to the power amplifier 32 is instantaneously increased, thus significantly contributing to the efficiency of the power amplifier 32. Accordingly, for example, when a voltage higher than an optimal voltage at the peak of the envelope signal I is applied, redundant power is supplied from a power supply, and the power efficiency is reduced. On the other hand, when a voltage lower than an optimal voltage at the peak of the envelope signal I is applied to the power amplifier 32, the amplification factor is reduced, and non-linear distortion is increased.